The present invention relates to an electromagnetic relay which is adapted to be driven by a.c. voltage.
In contrast to driving relays with d.c. voltage, the operation by means of a.c. voltage, particularly from the usual 110 or 220 V supplies, has been possible only with unpolarized relays of relatively large volume which can withstand coil losses of up to several watts at a coil volume of several cubic centimeters. Relays of this type are usually provided with a short-circuit ring to extend the drop-out time in such a manner that the relays neither de-energize nor oscillate during a.c. operation at for instance 50 Hz.
Polarized relays may be operated with a.c. voltage if a rectifier bridge or the like is used. The rectifying diodes act like quenching diodes, thereby extending the drop-out time of the relay by a factor of about 5. If it were desired to operate a relay of this type and of a reasonably small volume with an exciting energy of about 150 mW usual for polarized d.c. relays at an a.c. voltage of 220 V, such relay would require a coil resistance by far beyond what can be realized in practice.
If it is attempted to realize a maximum coil resistance in a modern relay, a maximum value of about 20 k.OMEGA. may be achieved by using the finest enameled copper wires which may be processed at reasonable expense. If such a relay were operated by an a.c. voltage of 220 V, 2.4 watts of electrical energy would be converted by the exciting coil into heat which would cause a rise in temperature of the relay by about 140.degree. C. In order to avoid such unallowable heating, an ohmic resistance used will have to be provided as a separate circuit element which causes not only an increase in the assembly costs but also losses of 1 to 2 watts, thus increased space requirement, which is particularly disadvantageous in printed circuit-board structures.
German Auslegeschrift No. 1 298 626 discloses a relay having its coil connected via a diode bridge and a capacitor to an a.c. voltage source, the capacitor serving to limit the exciting current. While this capacitor takes up only reactive energy, and thus produces no heat, it again forms a separate circuit element which requires even more space and is therefore particularly disadvantageous in modern circuits of high packing density.
German Offenlegungsschrift No. 2 749 732 discloses a relay having a coil consisting of two electrically separated and capacitively coupled windings to confine the period of current flow. This known relay, however, is intended to be driven by d.c. voltage, and it is the purpose of the capacity to ensure that the magnetic flux required for exciting the relay is reached only shortly, i.e. only at the moment the relay is switched on or switched over. Once the capacity is charged, it prevents any further d.c. current flow. If the relay known from German Offenlegungsschrift No. 2 749 732 referred to above were used in the circuit known from German Auslegeschrift No. 1 298 626 also referred to above, again no permanent electromagnetic flux through the relay would be achieved upon switching-on the a.c. voltage. Instead, the rectifying effect of the diode bridge in connection with the d.c. behaviour of the capacity between the two windings, only one short charging of this capacity resulting in one short electromagnetic flux would be obtained.